Here is link for you regarding herringbone design. the board length will be determined by the postioning of your blocking boards and the angel that you want.

www.deckplans.com/Images/pdf/herringbone.pdf

Good luck,
Deckman

I just finished putting in a herringbone surfaced deck. I would suggest that you use long measurements to lay out your 45 degree angles. For example, if your blocking boards are positioned 4' apart, measure 4' along the support board to get a good 45degree angle. If you use short measurements and/or are off on your angle, the pattern may 'walk' and you will find that the ends of your boards may eventually not fall on the blocking boards if the deck is long. The result is that you will have to pre-measure each board as you set them, which can be a pain. Taking time with the layout of the first boards will save you a lot of headaches down the road.

I used speed clamps to lay out the first boards of several courses of decking before I screwed the first decking boards down. That way I could check that the angles were correct and all the board lengths were equal.